P-doped electron conductive polymers, such as polypyrrole, polythiophene, polyaniline, exhibit a pseudo-capacitive behavior in their electro-chemistry; in other words, the quantity of electricity stored in the material is proportional to the applied voltage. This property disappears when the polymer loses its dopant and becomes insulating.
These polymers have very high specific capacitances, in the order of 200 F/g, which is much greater than obtains with carbon compounds of the active carbon type. These materials can thus be used to advantage as the active electrode material in super-capacitors.
Patents EP-A 300 330 and FR-A 89 10952 describe super-capacitors using polypyrrole. However it is found that the energy: stored in these super-capacitors (1/2 CV.sup.2) is not very high because of their low voltage (1.2 volts to 1.3 volts). It is the fall-off in mean potential of the electrodes which is responsible for this low voltage.
Taking the example of a super-capacitor whose electrodes are based polypyrrole (ppy) doped with ClO.sub.4.sup.- ions, the behavior during discharge can be schematized by the following reactions:
negative electrode
ppy+xClO.sub.4.sup.-.fwdarw.ppy.sup.x+ (ClO.sub.4.sup.-).sub.x +xe.sup.- PA1 ppy.sup.2x+ +(ClO.sub.4.sup.-).sub.2x +xe.sup.- .fwdarw.ppy.sup.x+ (ClO.sub.4.sup.-).sub.x +xClO.sub.4.sup.-
positive electrode
The negative electrode becomes doped and the positive electrode loses dopant.
It is observed that because of stray currents at high potential, a low Faradic yield of the electrodes or a large excess voltage, the negative electrode is "pushed" by the positive electrode towards a noncapacitive region. This leads to a loss of capacitance of the capacitor (loss of ultimate capacitance on the case of uni-polar cycling).